Interior permanent magnet (IPM) synchronous machines are commonly used as electric traction motors in hybrid electric and battery electric vehicles. IPM synchronous machines are energized via a DC power source, typically a rechargeable battery module, in conjunction with a current-controlled voltage source inverter. However, the permanent magnets used in the rotors of such machines can complicate the response taken by a motor controller to a detected motor or drive system fault.
For instance, at higher motor speeds, the rotating magnets can create a back electromagnetic force (EMF) voltage in the motor's stator windings. If switching within the voltage inverter is temporarily disabled in response to a detected fault, the back EMF voltage may cause diodes within the inverter to conduct, thereby allowing electrical current to flow back toward the battery module. This current flow condition is commonly referred to as an “UnControlled Generator” (UCG) state. UCG state characteristics may include the presence of a relatively large amount of regenerative braking torque acting on the machine, as well as conduction of significant electrical current back to the battery module.
To combat this result, controllers of IPM-type synchronous machines may execute, for an example three-phase machine, a three-phase short as a fault condition remedial action. Semiconductor switches of the voltage inverter are turned on simultaneously to cause the three-phase short, which in turn prevents electrical current from flowing to or from the battery module. At higher motor speeds, braking torque is relatively low, which is favorable for traction drive applications. The machine impedance will limit the motor currents during a three-phase short operation. Additionally, stator current approaches the characteristic current of the machine for most motor speeds. However, while a three-phase short remains a viable fault response, conventional approaches for implementing the three-phase short remain less than optimal.